Convertion of tabular data to JSON in Redshift - amazon-redshift

Question

I am unable to figure out how to convert tabular data to JSON format and store it in another table in Redshift. For example, I have a "DEMO" table with four columns: pid,stid,item_id,trans_id.
For each combination of pid,stid,item_id there exist many trans_ids.
pid stid item_id trans_id :
1 , AB , P1 , T1
1 , AB , P1 , T2
1 , AB , P1 , T3
1 , AB , P1 , T4
2 , ABC , P2 , T5
2 , ABC , P2 , T6
2 , ABC , P2 , T7
2 , ABC , P2 , T8
I want to store this data in another table called "SAMPLE" as:
pid stid item_id trans_id
1 , AB , P1 , {"key1":T1, "key2":"T2" "key2":"T3" "key2":"T4"}
2 , ABC , P2 , {"key1":T5, "key2":"T6" "key2":"T7" "key2":"T8"}
I am unable to figure out how to load the data from "DEMO" to "SAMPLE" in JSON format only for column "trans_id" using a SQL query in Redshift. I don't want to use any intermediate files.

Answer 1

There is LISTAGG aggregate function that allows you to concatenate text values within groups. It allows the effective construction of JSON objects:
SELECT
pid
,stid
,item_id
,'{'||listagg(
'"key'||row_number::varchar||'":'||trans_id::varchar
,',') within group (order by row_number)
||'}'
FROM (
SELECT *, row_number() over (partition by pid,stid,item_id order by trans_id)
FROM "DEMO"
)
GROUP BY 1,2,3;
As a side note, in this particular case an array of transaction IDs might work better, you'll be able to request the element of a specific order easily without using keyN key:
WITH tran_arrays as (
SELECT
pid
,stid
,item_id
,listagg(trans_id::varchar,',') within group (order by trans_id) as tran_array
FROM "DEMO"
GROUP BY 1,2,3
)
SELECT *
,split_part(tran_array,',',1) as first_element
FROM tran_arrays;

Answer 2

Very similar to the existing Answer however slightly different. This example is also run out of an Oracle Database. I put the work into it and felt like sharing in case it may help someone else out.
/* Oracle Example */
WITH demo_data AS
(
SELECT 1 AS pid, 'AB' AS stid, 'P1' AS item_id, 'T1' AS trans_id FROM dual UNION ALL
SELECT 1 AS pid, 'AB' AS stid, 'P1' AS item_id, 'T2' AS trans_id FROM dual UNION ALL
SELECT 1 AS pid, 'AB' AS stid, 'P1' AS item_id, 'T3' AS trans_id FROM dual UNION ALL
SELECT 1 AS pid, 'AB' AS stid, 'P1' AS item_id, 'T4' AS trans_id FROM dual UNION ALL
SELECT 2 AS pid, 'ABC' AS stid, 'P2' AS item_id, 'T5' AS trans_id FROM dual UNION ALL
SELECT 2 AS pid, 'ABC' AS stid, 'P2' AS item_id, 'T6' AS trans_id FROM dual UNION ALL
SELECT 2 AS pid, 'ABC' AS stid, 'P2' AS item_id, 'T7' AS trans_id FROM dual UNION ALL
SELECT 2 AS pid, 'ABC' AS stid, 'P2' AS item_id, 'T8' AS trans_id FROM dual
)
, transformData AS
(
SELECT pid, stid, item_id, trans_id, rownum AS keyNum FROM demo_data
)
SELECT pid, stid, item_id
, '{'||
LISTAGG(CHR(34)||'key'||keynum||CHR(34)||':'||CHR(34)||trans_id||CHR(34), ' ')
WITHIN GROUP (ORDER BY pid)
||'}' AS trans_id
FROM transformData
GROUP BY pid, stid, item_id
;
Output will look like this: